Conventionally, a Group III nitride semiconductor represented by a compositional formula AlxGayInzN (0≦X, Y, Z≦1; X+Y+Z=1) has been used in, for example, a Group III nitride semiconductor light-emitting device emitting visible light of short wavelength (see, for example, Book written and edited by Isamu AKASAKI, “Group III-V Compound Semiconductor,” published by Baifukan Co., Ltd., Chapter 13, May 20 (1994), first edition). Specifically, the semiconductor is employed to form a p-type cladding layer and an n-type cladding layer in a light-emitting diode (LED) (see the same Book as mentioned above). In a semiconductor light-emitting device, such as an LED and a laser diode (LD), in order to attain high emission efficiency, a light-emitting portion is generally formed of a pn-junction type double hetero (DH) structure (see the same Book as mentioned above). The light-emitting portion of the DH structure refers to a junction structure portion including a light-emitting layer, a p-type cladding layer and an n-type cladding layer, each cladding layer forming a junction with a surface of the light-emitting layer.
In Group III nitride semiconductor light-emitting devices having a pn-junction type DH structure, a light-emitting layer is generally formed of a Group III-V compound semiconductor material, such as gallium indium nitride (GayInzN in which 0≦Y, Z≦1 and Y+Z=1) or gallium nitride phosphide (GaN1−QPQ in which 0≦Q≦1). Conventionally, a p-type cladding layer for sandwiching the light-emitting layer is generally formed of AlxGayN (0≦X, Y≦1; X+Y=1) (see, for example, Japanese Patent No. 2713094). However, in order to obtain a low-resistance p-type Group III nitride semiconductor layer suitable for a barrier layer, doping of the semiconductor layer with a Group II element such as magnesium (Mg) during formation of the layer (a) (see, for example, Japanese Patent No. 2778405) and heat treatment for removing hydrogen atoms from the layer (b) (see, for example, Japanese Patent No. 2540791) must be performed.
Meanwhile, a low-resistance p-type conductive layer is also produced from boron phosphide (BP) to which no impurity is intentionally added; i.e., undoped BP (see, for example, T. Udagawa et al., Technical Digest of 5th. Int. Conf. Nitride Semiconductors, p. 431). Recently, a technique for forming a Group III nitride semiconductor light-emitting device having a pn-junction type DH structure from boron phosphide has been disclosed (see, for example, U.S. Pat. No. 6,069,021).
However, a p-type boron phosphide continuous film cannot be reliably grown on an n-type Group III nitride semiconductor layer. Namely, the formed boron phosphide layer fails to have continuity due to a large number of pits present in the Group III nitride semiconductor layer, thereby disturbing smooth current flow due to its discontinuity and exhibiting high electric resistance. Thus, when such a discontinuous boron phosphide layer is employed as a cladding layer, the fabricated Group III nitride semiconductor light-emitting device has a small light-emitting area attributed to, for example, failing to diffuse device operation current over the layer.
The present invention has been accomplished in order to solve the problems involved in the aforementioned conventional techniques. Thus, an object of the invention is to propose a stacked structure for reliably growing, on an n-type Group III-V compound semiconductor layer, a p-type boron phosphide layer having excellent continuity. Another object of the invention is to provide a compound semiconductor light-emitting device having a pn-junction type heterostructure (hereinafter referred to as “pn-heterojunction compound semiconductor light-emitting device”) that contains a p-type boron phosphide layer and which device is formed by use of the stacked structure.